pgsql: Tweak tests to support Python 3.7

Tweak tests to support Python 3.7

Python 3.7 removes the trailing comma in the repr() of
BaseException (see <https://bugs.python.org/issue30399&gt;), leading to
test output differences. Work around that by composing the equivalent
test output in a more manual way.

Branch
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REL9_3_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e7f90471590025ccb522562bec8513238805395b

Modified Files
--------------
src/pl/plpython/expected/plpython_subtransaction.out | 6 +++---
src/pl/plpython/expected/plpython_subtransaction_0.out | 2 +-
src/pl/plpython/expected/plpython_subtransaction_5.out | 2 +-
src/pl/plpython/sql/plpython_subtransaction.sql | 2 +-
4 files changed, 6 insertions(+), 6 deletions(-)